2. Transport through the root

Question 1

Why is water needed in plants

Answer 1

• Mineral ions & sugars are transported in aqueous solution
• Water is a raw material of photosynthesis
• Cooling effect (by transpiration)
• Turgor pressure - hydrostatic skeleton

Question 2

Simple overview of water movement through plants

Answer 2

1. Water uptake near root tips (HIGH WATER POT)
2. Water enters xylem
3. Water moves up xylem
4. Water moves from xylem to leaf cells
5. Evaporation of water into leaf air spaces
6. Transpiration of water vapour through open stomata into air (LOW WATER POT)
   (see s7)

Question 3

What is water potential

Answer 3

The tendency for water molecules to move within & between cells
Water moves from region of HIGHER water pot to region of LOWER water pot

Question 4

plasmolyzed. flaccid. turgid

Question 5

What has the highest water potential

Answer 5

Pure water

Question 6

How is water potential of a solution LOWERED

Answer 6

Presence of solutes lowers the water pot of a solution

Question 7

What does having water potential of 0 mean

Answer 7

Maximum water potential
- Therefore all solutions have a water pot less than 0 (-ve number)

Question 8

Uptake of water process

Answer 8

A passive process
By osmosis

Question 9

Uptake of minerals

Answer 9

Can be passive or active
By diffusion or active transport (respectively)

Question 10

How does water potential inside the cell affect movement of water

Answer 10

• If water pot inside the cell is low (higher salt concent) water will move in by osmosis
• If water pot inside the cell is high (low salt concent) water will move out by osmosis

Question 11

What happens in low external water potential

Answer 11

Water moves out of the cell
- Plants can survive this for short periods, as they can shrink the cell membrane away from the cell water
- The cell is said to be plasmolysed

Question 12

s14 for dia of movement of water from root hair to xylem

Question 13

Adaptations of root hair cells

Answer 13

• Very thin surface layer (wall & plasma membrane) - so diffusion & osmosis can happen quickly
• Microscopic in size - meaning they can penetrate easily between soil particles
• Large SA:V ratio
• Concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water & the cell

Question 14

see s18… for structure of a root dia

Question 15

What are casparian strips

Answer 15

Prevents movement of water across the cell wall

Question 16

Water movement pathways (3)

Answer 16

Water moves through the roots cells & into xylem tube by 3 pathways:
- Symplast pathway
- Apoplastpathway
- Vacuolar pathway
These allow plant to get water in as fast as possible

Question 17

Symplast pathway

Answer 17

Movement of water through the living spaces of the cell - cytoplasm
- Enters cells through the plasmodesmata
- Each cell further away from the roots has a lower water pot so water is drawn through the plant
- Water moves by osmosis into the cell & between cells through the plasmodesmata

Question 18

Apoplast pathway

Answer 18

Movement of water through the cell wall & intracellular spaces
- Cohesive & tension forces acting on the cell walls pulls the water up the plants
- This is the fastest movement of water
- Water moves by diffusion as its not crossing a partially permeable membrane (simply cell to cell or through intracellular spaces)

Question 19

What is the reason for the presence of the Casparian strip

Answer 19

Not fully understood but scientists think that:
- this may help the plant control which mineral ions reach the xylem
- it plays a part in increasing root pressure

Question 20

What happens to the Casparian strip as the plant ages

Answer 20

Thickens (as more suberin is deposited) except in cells called the passage cells, allowing for further control of the mineral ions

Question 21

Vacuolar pathway

Answer 21

The same as the symplast pathway when the water moves through the cells vacuoles in addition to the cytoplasm
- This is the slowest route

Question 22

see s32 for dia of apoplast, symplast, vacuolar pathways

Question 23

What is the Casparian strip

Answer 23

An impermeable layer of suberin - a waxy material
As a result, all water in apoplast pathway is forced into symplast pathway

Question 24

Where is the Casparian strip

Answer 24

The endodermis contains the casparian strip

Question 25

Explain the casparian strip (+following events)

Answer 25

• When water reaches the endodermis of root, its path is blocked
• The endodermis has a waterproof, impermeable layer - Casparian strip in its walls. This is bc of the waxy layer of suberin in walls of endodermal cells
• In order to cross the endodermis, water that has been moving through cell walls (in apoplast pathway), must now move through the cell surface membrane & into the cytoplasm (forced into symplast pathway)
• This allows the partially percale plasma membrane of the cells can control water enters the xylem tissue
• This is important as the cell surface membrane can remove any toxic solutes from the soil, only allowing necessary water molecules/mineral ions to enter

Question 26

Evidence for active transport in root pressure

Answer 26

• Effect of Cyanide
• Effect of Temperature
• Reactant availability
• Guttation

Question 27

Evidence for active transport in root pressure: Effect of Cyanide

Answer 27

Cyanide stops the mitochondria from working, therefore root pressure decreases

Question 28

Evidence for active transport in root pressure: Effect of Temperature

Answer 28

Root pressure increases as temp increases & decreases as temp decreases, suggesting an enzyme controlled chemical reaction

Question 29

Evidence for active transport in root pressure: Reactant availability

Answer 29

If oxygen levels or respiratory substrate levels drops, root pressure decreases

Question 30

Evidence for active transport in root pressure: Guttation

Answer 30

Sap & water will move out of cut stems, suggesting they are actively pumped out & not drawn up by transpiration